Two Apps running with cross communication between them

I need a way to run two apps and have them communicate with each other. First App will have control buttons and when pressed must be able to send this to the second App so it will respond to the button clicks from the first App. The second App must be able to send results back to the first App.

The Apps are running in real-time.

I don't need the code (although small examples would be nice); just some ideas.

Like in the other thread: look up activex-dll/exe. Set a reference in your 1st app to the second app. Clicking on a button in your 1st app calls a public method of your public class in your 2nd app, which processes your requirement and when finished fires its public event, which can be received in your 1st app